Long Term Evolution (LTE) Advanced is a mobile telecommunication standard proposed by the 3rd Generation Partnership Project (3GPP) and first standardised in 3GPP Release 10. In order to provide the peak bandwidth requirements of a 4th Generation system as defined by the International Telecommunication Union Radiocommunication (ITU-R) Sector, while maintaining compatibility with legacy mobile communication equipment, LTE Advanced proposes the aggregation of multiple carrier signals in order to provide a higher aggregate bandwidth than would be available if transmitting via a single carrier signal. This technique of Carrier Aggregation (CA) requires each utilised carrier signal to be demodulated at the receiver, whereafter the message data from each of the signals can be combined in order to reconstruct the original data. Carrier Aggregation can be used also in other radio communication protocols such as High Speed packet Access (HSPA).
FIG. 1 illustrates two main variations of CA on frequency-amplitude graph 100. The graph areas 102, 104, 106 and 108 are representative of signals modulated at carrier frequencies fC1, fC2, fC3 and fC4 respectively. Carrier frequencies fC1, fC2, fC3 and fC4 are associated with a set of adjacent channels used in a communication scheme, such as LTE, which has been assigned for use in the displayed section of the frequency spectrum. Each communication channel may be separated by guard bands, which are small unused sections of the frequency spectrum designed to improve the ease with which individual signals can be selected by filters at the receiver by reducing the likelihood of interference between signals transmitted in adjacent channels.
In a first communication instance, data is transmitted to a user terminal using the aggregation of signals 102 and 104 modulated at carrier frequencies fC1 and fC2 respectively. This is an example of contiguous CA, where data is transmitted at carrier frequencies that are adjacent in the frequency spectrum. In a second communication instance, data is transmitted to a user terminal using the aggregation of signals 106 and 108 modulated at carrier frequencies fC3 and fC4 respectively. This is an example of non-contiguous CA, where data is transmitted at carrier frequencies that are separated by one or more intermediate carrier frequencies (in this case fC1 and fC2) not used in the communication instance. In some non-contiguous CA arrangements, the aggregated signals may be in entirely different frequency bands.
Several radio communication schemes, including LTE, use quadrature modulation to increase the data density of a single frequency channel by transmitting a second message which is modulated with a carrier that is 90° out of phase with respect to a first message. These two message components are termed the in-phase (I) and quadrature (Q) components respectively. A common method for processing a quadrature modulated signal uses a receiver arrangement known as a Direct Conversion Receiver (DCR).
FIG. 2 illustrates a schematic diagram of an exemplary DCR as known in the art. A wide range of radio communication signals are received at antenna 200, which is tuned by a wide bandwidth pre-selection filter 202. The filtering behaviour of pre-selection filter 202 may arise out of the physical and electromagnetic characteristics of the antenna design, perhaps due to optimisation for the frequency band of the desired telecommunication application. Pre-selection filter 202 may also include one or more tuned circuits, which are used to remove frequency components of the input that are far outside the intended range of reception frequencies. The received signal is typically of very small amplitude, and requires amplification by low noise amplifier 204 before further processing can be performed. Low noise amplifier 204 must operate at high frequencies (at least the transmission frequency of the signal the receiver is intended to receive), commonly referred to as radio frequency, and also provide very low noise insertion due to the weakness of the incoming signal.
In order to select the appropriate signal from the many signals received at antenna 200, the received input must be filtered. However, the high selectivity of the filter profile that would be required to isolate one signal at radio frequency makes filtering at this stage either unrealistic (given the manufacturing tolerances of commonly available components) or undesirably expensive. Hence, before signal selection can occur, the frequency of the desired signal must be down-converted by mixing the input signal with a signal generated by local oscillator 206. A direct conversion receiver converts the desired signal directly to baseband frequency by mixing it with a local oscillator signal of the same frequency as the carrier frequency of the desired signal. This has the effect of translating the desired signal to be centred on zero frequency, as described in further detail below with respect to FIG. 3.
In order to extract both the I and Q components, the input signal must be mixed with both in-phase and quadrature shifted versions of the local oscillator signal, which are generated by quadrature generator 208. The exact phase of the received signal cannot be predicted due to the unknown phase shift that will occur during transmission. Hence, the local oscillator must synchronise with the received signal in order to ensure the necessary phase relationship. This synchronisation may be achieved by establishing a phase reference, for example by using a phase locked loop (PLL) or by rotating the signal after down-conversion by digital means. The input signal is mixed with the in-phase local oscillator signal by mixer 210, and with the quadrature phase local oscillator signal by mixer 212. Mixers 210 and 212 perform multiplication between the input signal and the appropriate local oscillator signal in order to achieve the required frequency down-conversion.
The desired I and Q components can then be isolated using low pass filters 214 and 216 respectively, which are used to suppress unwanted frequencies associated with signals adjacent in adjacent channels etc. Finally, Analogue to digital converters (ADCs) 218 and 220 convert the I and Q components into binary representations of the I and Q message data 222 and 224. Once in the digital domain, further processing can be performed on the I and Q data, including recombination of the components to form the original data message.
With minimal modification, a DCR can also be used to receive a specific CA configuration, defined in 3GPP TR 36.807 as Carrier Aggregation bandwidth class C, which involves the aggregation of two adjacent frequency channels. Signals 102 and 104 of FIG. 1 are an example of CA according to such a configuration.
FIG. 3 illustrates the operation of conventional DCR hardware when used to receive data transmitted via two adjacent signals on frequency-amplitude graph 300. Signals 302 and 304 are modulated at carrier frequencies fC1 and fC2 respectively and used in a contiguous CA configuration. Mixing a signal having carrier frequency fC with a local oscillator signal of frequency fLO has the effect of translating the signal to be centred on new frequencies at fLO−fC and fLO+fC. Frequency down-conversion as preformed in receiver circuits utilises the signal translated at fLO−fC. By mixing the input with local oscillator signal 306 having a frequency halfway between fC1 and fC2, signals 302 and 304 are both frequency down-converted and translated to become centred on the same frequency.
Signal 302 is translated to be centred on frequency fLO−fC1 as shown by arrow 308. Signal 304 is translated to be centred on frequency fLO−fC2 which is the same as −(fC2−fLO), where the negative sign indicates a 180° phase shift. Hence signal 304 is phase inverted and translated to be centred on frequency fC2−fLO which, due to the choice of frequency of the local oscillator signal 306, is equivalent to fLO−fC1, as shown by arrow 310. The result of these translations is to generate a composite signal 312 made up of signal 302 and a phase inverted signal 304 centred on frequency fLO−fC1. The DCR hardware then acts as a low intermediate frequency (IF) receiver for both carriers in order to produce a combined I component of both signals and a combined Q component of both signals. The individual I and Q components of each signal can then be isolated in subsequent processing due to the phase inversion between them, for example by performing a Fourier Transform operation, and the original data can be reconstructed from the four message components.
Problems arise when the signal strengths of the two signals arriving at the receiver are substantially different. This could occur for several reasons, for example due to the different propagation characteristics of the different frequency carrier waves. Further, different carrier frequencies may be associated with different cell coverage areas and different transmitter directivity. Additionally, the coverage of one of the frequencies could be augmented through the provisioning of frequency selective repeaters, thereby raising the signal strength of one signal relative to the other.
FIG. 4 illustrates the operation of conventional DCR hardware when used to receive data transmitted via two adjacent signals having a relative signal strength imbalance on frequency amplitude graph 400. Adjacent signals 402 and 404 are modulated at carrier frequencies fC1 and fC2 respectively; however when received at the receiver, the signal strength of signal 402 is significantly lower than the signal strength of signal 404. As described above, when mixed with local oscillator signal 406 of frequency fLO, both signals are frequency down-converted and translated to be centred on frequency fLO−fC1 as shown by arrows 408 and 410.
The result of these translations is to generate a composite signal 412 made up of signal 402 and a phase inverted signal 404 centred on frequency fLO−fC1. Due to imperfections such as component mismatch in the mixer and low pass filter hardware, or the quality of quadrature signals from the local oscillator, the image-reject ratio (IRR) of the receiver for isolating the I and Q components of the combined signal will not be infinite. As the signal strength imbalance between the two signals increases, a higher receiver I-Q performance is required in order to successfully isolate the signal components. Due to these imperfections, there is a limit to the signal strength imbalance between two contiguous CA carriers that can be successfully processed by a single conventional DCR receiver path, after which the translated relatively low strength signal is overpowered by the presence of the relatively higher strength one.
FIG. 5 schematically illustrates an alternative known hardware arrangement for receiving data transmitted via two adjacent signals having a relative signal strength imbalance. In FIG. 5, a dedicated receiver path is provided for the reception of each of the two CA signals. A first receiver path contains antenna 500; pre-selection filter 502; low-noise amplifier 504; local oscillator 506; quadrature generator 508; mixers 510 and 512; low-pass filters 514 and 516; and ADCs 518 and 520. A second receiver path contains antenna 500; pre-selection filter 502; low-noise amplifier 526; local oscillator 528; quadrature generator 530; mixers 532 and 534; low-pass filters 536 and 538; and ADCs 540 and 542. The operation of antenna 500; pre-selection filter 502; low-noise amplifiers 504 and 526; quadrature generators 508 and 530; mixers 510, 512, 532 and 534; low-pass filters 514, 516, 536 and 538; and ADCs 518, 520, 540 and 542; are the same as described previously in relation to FIG. 2. However, local oscillators 506 and 528 are configured to produce different frequencies by operating at the carrier frequency of the signal intended to be processed by their respective paths. In this manner, the two receiver paths operate as two individual DCRs, one arranged to receive each carrier signal.
Due to the use of independently configurable local oscillators, this method is more commonly used for non-contiguous carrier aggregation, where the two signals may be transmitted at very different carrier frequencies and the single DCR operating as a low-IF receiver as described previously is not appropriate. However, when applied to a contiguous carrier aggregation configuration, the two local oscillators must operate at very similar frequencies. In single integrated circuit deployments, the configuration may suffer from local oscillator pulling due to difficulties in sufficiently isolating the two local oscillators from one another. The effect of this is to cause instabilities in the generated signals as the two operating frequencies tend towards each other, thereby impeding the successful operation of the receiver. Further, this arrangement has increased silicon area and power consumption costs when compared to the single DCR arrangement described with reference to FIG. 2, which make it a less desirable solution. Hence, it is an objective of the present invention to provide improved receiver hardware, capable of effectively receiving data transmitted via aggregated carrier signals.